Electrosurgical instruments are a type of surgical instrument used in many surgical operations. Electrosurgical instruments apply electrical energy to tissue in order to treat tissue. An electrosurgical instrument may comprise an instrument having a distally-mounted end effector comprising one or more electrodes. The end effector can be positioned against tissue such that electrical current is introduced into the tissue. Electrosurgical instruments can be configured for bipolar or monopolar operation. During bipolar operation, current is introduced into and returned from the tissue by active and return electrodes, respectively, of the end effector. During monopolar operation, current is introduced into the tissue by an active (or source) electrode of the end effector and returned through a return electrode (e.g., a grounding pad) separately located on a patient's body. Heat generated by the current flow through the tissue may form hemostatic seals within the tissue and/or between tissues and thus may be particularly useful for sealing blood vessels, for example. The end effector of an electrosurgical instrument sometimes also comprises a cutting member that is moveable relative to the tissue and the electrodes to transect the tissue.
Energy applied by an electrosurgical instrument can be transmitted to the instrument by a generator. The generator may form an electrosurgical signal that is applied to an electrode or electrodes of the electrosurgical instrument. The generator may be external or integral to the electrosurgical instrument. The electrosurgical signal may be in the form of radio frequency (“RF”) energy. For example, RF energy may be provided at a frequency range of between 100 kHz and 1 MHz. During operation, an electrosurgical instrument can transmit RF energy through tissue, which causes ionic agitation, or friction, in effect resistive heating, thereby increasing the temperature of the tissue. Because a sharp boundary may be created between the affected tissue and the surrounding tissue, surgeons can operate with a high level of precision and control, without sacrificing un-targeted adjacent tissue. The low operating temperatures of RF energy may be useful for removing, shrinking, or sculpting soft tissue while simultaneously sealing blood vessels. RF energy may work particularly well on connective tissue, which is primarily comprised of collagen and shrinks when contacted by heat. In some cases, the instrument may also be configured to apply ultrasonic energy to create homeostasis. The generator may be configured to transmit energy which is converted into ultrasonic vibrations at the end effector. The energy transmitted to the tissue may similarly cause resistive heating through the ultrasonic vibrations.
During the application of the energy to tissue, the impedance of the tissue indicates the condition of the tissue. After a certain amount of energy applied, the impedance of the tissue dramatically increases and reduces the effectiveness of the further energy applied in the sealing procedure. Furthermore, as the tissue impedance approaches this threshold level where further energy applied is no longer effective, certain chemical processes in the tissue occur that would be desirable to be controlled better. The period of time under which the tissue responds to the sealing energy is sometimes referred to as the “bathtub region,” based on the shape of the level of impedance over time at which the tissue effectively responds to the sealing energy. There is a need therefore to better control the rise of impedance levels in the tissue and to prolong the period under which the tissue still responds (e.g., prolong the “bathtub region”) to applied energy during sealing procedures. While several devices have been made and used, it is believed that no one prior to the inventors has made or used the device described in the appended claims.